Conventional sewage purification apparatuses use a concrete reservoir in a spacious land or building, wherein an apparatus to culture aerobic bacteria is equipped in the reservoir water to which bacteria are continuously supplemented. Simultaneously, air is sent by a pump in the water for activation. Contaminated water diluted by several-fold water is contacted by bacteria so that contaminants included in the water is digested by bacteria and purified. However, it takes several months and needs large equipment and high costs. It also needs an enormous land and apparatus for complete purification. Because of this, contaminated water has undesirably been drained in rivers after diluted by several-fold water, resulting in polluted rivers, lakes, swamps and seas. A filtering apparatus using sands, activated carbon, membranes or the like, is much expensive and very uneconomical.
In the past, various kinds of single flocculants have been used to purify water by cohering and removing sludge and inorganic matters from contaminated water. Aluminum sulfate, ferric chloride, alum, polymer flocculants and the like have been used alone as the flocculant. When contaminated water was mixed and stirred with a single flocculant and left to settle, only small stones and sands cohere and settle on the bottom of the container after 6 to 24 hours, whereas the water would not become transparent. In civil constructions, the process was repeated until the contaminated water became transparent. However, it was impossible to remove impurities dissolved in the water even spending 30% of the construction costs.
Contaminated water generated in construction sites was sent by a pump into a large tank installed on the highest mountain or hill nearby, wherein the water was stirred with a single flocculant and left for 6 to 24 hours to separate inorganic matters by cohesion-sedimentation. Then supernatant, though still translucent, was transferred to another purification tank installed at a lower level, wherein a flock was stirred again and left for 6 to 24 hours, thereafter the supernatant was transferred to another tank installed on the hill at a lower level. This process was repeated several times.
Said process was repeated more than 6 to 10 times, and transparent water was finally drained into the river. Even now, ferric chloride, alum or the like is used in construction sites. (However, BOD and COD of the contaminated water or purified water are not taken into account.) Therefore, even if the water became transparent, it was still contaminated.
Several features in using a single flocculant are as follows:
(1) Flocculation and purification effects are low and slow; PA1 (2) No deodorant effect can be expected; PA1 (3) No sterilization effects can be expected; PA1 (4) No decolorant effects can be expected; PA1 (5) For the purpose of merely obtaining clear water from contaminated water, it costs about 30% of total cost in a engineering work. The process includes transferring supernatant or overflow to a lower stage vessel for 5 to 10 times which is done every 6 to 24 hours and eventually clear water is obtained to be flown into a river. PA1 (6) The process is not capable of removing materials dissolved in water and thus such materials remain in clear water to be flown into a river.
It is understood in the academic society and industries that contaminants included in contaminated water are unable to be purified by separation and removal using any kinds of flocculants. Therefore, in water treatment plants and sewage purification plants, solid materials and inorganic matters in the sewage are first removed by slow sedimentation, filtering, or by a slow sedimentation-separation using single flocculant (ferric chloride) mixed and stirred. Then after a slow sedimentation-separation, it is contacted with bacteria for 1 to 5 days by means of the activated-sludge process, whereby invisibly small bacteria are cultivated by giving air to digest it. This process calls for an enormous land, equipment, costs and time. Nevertheless, perfect purification is impossible. Therefore, contaminated water has been drained into rivers after adjusting its pH and filtered.
In dung processing, dung was contacted with bacteria for 5 to 7 days by the activated sludge process, then filtered through high polymer membranes, which cost 20 million Yen per cubic meter. This way, however, COD could be lowered to only 90 to 95 mg/liter. Thereafter, it was mixed with ferric chloride to settle contaminants included in it, then let it through a layer of activated carbon to make transparent water with COD of 30 mg/liter before drained into the river. This was the limit but was expensive; the cost of the purification equipment was 20 million yen per ton, while the processing costs were 2,500 to 6,000 yen per ton.